This invention relates generally to gamma cameras and more particularly to a circuit for synchronizing the outputs of multiple photomultiplier tubes for processing as a group.
One of the limiting characteristics of a nuclear imaging system is that of dead time, i.e., the time during which the system processes a single event and is therefore not available to process a succeeding event. A common way to reduce the dead time of a system is to provide redundancy by way of buffers in those parts of the processing cycle which are the slowest, such as, for example, in the energy or spatial correction functions. There are also other techniques that are applied to speed up the signal processing functions so as to thereby reduce the dead time of the system. Despite the use of these techniques, however, there are practical considerations that limit the speed at which the counts can be received and processed. This practical limit is generally considered to be around 200,000 counts per second for a typical conventional gamma camera. One reason for this limitation is that an array of photomultiplier tubes is only capable of receiving or processing one event at a time. That is, if two events occur simultaneously, or near simultaneously, at different locations anywhere in the array, then at least one of the events, and possibly both of the events, will be discarded. The usual occurrence will be for the event which is first to occur or first to be processed to be used and the other one to be discarded.
It may happen that both of the otherwise valid events are discarded because they are interpreted as a single event. For example, if two displaced events occur simultaneously, they may be viewed collectively as a single, brighter event at a position intermediate the two. In such case, the window-level discrimination may cause the event to be discarded because of its brightness exceeding the allowed threshold level. Such an elimination of two counts is therefore significant in reducing the count rate of the system.
Another undesirable phenomenon which might occur is that of interpreting two bad events as a single good event. This can occur where a pair of displaced, weak events are collectively seen as a single, brighter event located at a position intermediate the two. If that brighter event is within the prescribed brightness window, then it will be erroneously counted as a valid event to thereby contribute to the inaccuracy of the process.